1. Field of the Invention
The present invention relates to a glow plug energization control apparatus for controlling energization to a glow plug for assisting the starting of an internal combustion engine, and a glow plug energization control method.
2. Description of the Related Art
A glow plug generally uses a resistance heater. This glow plug is constructed by attaching the resistance heater to a main body metal fitting, is attached to an engine block of a diesel engine so that a tip of the resistance heater is positioned in a combustion chamber, and is used.
As an apparatus for controlling the energization to such a glow plug, a glow plug energization control apparatus is known. In a conventional glow plug energization control apparatus, when a key switch is put in an on position, the energization to the glow plug is controlled so that the temperature of the resistance heater is raised to a first target temperature (for example, 1000° C.) sufficient to start an engine, and a large electric power is supplied to the glow plug. The step as stated above is generally called a pre-glow or a pre-glow step. In the glow plug which can quickly heat, the temperature of the resistance heater can be raised up to the first target temperature in several seconds. Next, after the temperature of the resistance heater reaches the first target temperature, the energization to the glow plug is controlled so that the temperature of the resistance heater is kept at a second target temperature (for example, 900° C.) for a predetermined period (for example, 180 seconds), and a low electric power is supplied to the glow plug. The state as stated above is generally called an after-glow or an after-glow step. Before the engine is started, the temperature of the resistance heater is kept at the sufficiently high temperature so that the engine can be started at any time, and after the engine is started, warm-up in a combustion chamber of the engine can be accelerated, and it is possible to prevent the occurrence of diesel knock, and to suppress the occurrence of noise and white smoke, the exhaustion of HC composition, and the like.
As documents relating to such a technique, for example, JP-A-56-129763 and JP-A-60-67775 can be named.
However, in the conventional glow plug energization apparatus, there has been a phenomenon in which immediately after the pre-glow step is shifted to the after-glow step, the temperature of the resistance heater becomes once lower than the second target temperature to be kept. It is considered that this is caused since even if the temperature of the resistance heater reaches the first target temperature by the pre-glow step, the temperature of the surrounding of the resistance heater, such as an engine block and a main body metal fitting, is low and these do not yet reach the steady state, so that the temperature of the resistance heater is much absorbed by these. In the state where the temperature of the resistance heater is dropped as stated above, even if a key switch is put in a start position to start cranking, the engine is hard to start.
Besides, when the key switch is put in the start position in the after-glow step and the cranking of the engine is started, also in this case, there has been a phenomenon in which the temperature of the resistance heater becomes lower than the second target temperature to be kept. First, it is considered that this is caused since the resistance heater is cooled by external factors such as burning spray and swirl. Besides, second, it is considered that this is caused since during the cranking, an excessive electric power is required for the cranking, the voltage of a battery is remarkably lowered. Especially, immediately after the shift to the after-glow step, since heat conduction from the resistance heater to the surrounding also occurs as stated above, there has been a problem that the temperature of the resistance heater is remarkably lowered, and the startability of the engine is inferior.